GEOMAGNETIC-ACTIVITY DURING SOLAR-CYCLE 9 (1844-1856)

Ten-minute values of declination observations carried out at the Helsi nki observatory were used to analyze magnetic activity and storminess during 1844-1856 spanning solar cycle 9. Three-hour K-indices and dail y Ak-amplitudes were derived numerically from the data and compared wi th corresponding indices from the present Nurmijarvi magnetic observat ory nearby. Helsinki magnetic activity data are based on homogeneous a nd reliable observations being thus utilizable for extending long acti vity series (like aa since 1868 onwards) backwards in time. Magnetic a ctivity during 1844-1856 has been, on the average, on the same level a s the activity at Nurmnijarvi during the last four sunspot cycles in 1 953-1992 but the occurrence of very large magnetic storms has been mor e frequent at Nurmijarvi. There have been about 300 magnetic storm day s (Ak greater-than-or-equal-to 30) during cycle 9. The distribution of storm occurrence was double-peaked within the cycle: in the rising (1 847-48) and falling (1851-54) parts of the sunspot cycle substantially more magnetic storms appeared than during the years in between.